The present invention is in a process for reducing higher metal oxides to lower metal oxides by treatment with a carbonaceous reducing agent.
Ores which contain metals, such as Fe, Ni, Mn, in the form of higher oxides must sometimes be subjected to a reducing treatment to obtain the metals in a lower oxide form. This is particularly required in processes of producing iron-nickel alloys from iron-nickel ores.
Poor ores, such as lateritic ores, must increasingly be used to meet the demand of industry for nickel, particularly in alloys with iron. Most of the poor ores contain Fe and Ni in a ratio such that a complete reduction of both metals, and a separation of the gangue in a molten state as slag, would result in a ferroalloy which is so poor in nickel that it would not be commercially acceptable.
For instance, in an ore containing 30% Fe and 2% Ni the ratio of Fe to Ni is 15:1. However, the Fe:Ni ratio in commercial ferroalloys is not in excess of 4:1, which means that they contain at least 20% nickel.
For this reason the processing of such ores includes a preliminary reduction, by which they are reduced, as closely as possible, to an FeO state and a succeeding melting process, in which metallic iron is produced by a further reduction only in that amount which is permissible for the desired ferroalloy. The remaining iron oxide is slagged.
On a commercial scale, the preliminary reduction is effected in a rotary kiln and coal is used as the reducing agent. A problem arising in connection with the preliminary reduction in a rotary kiln is that the iron oxide must be reduced by the preliminary reduction exactly to a prescribed degree and that the discharged material must contain surplus solid carbon only in an amount which is still permissible for the further reduction in the melting process to the desired content of metallic iron. Formation of metallic iron by the preliminary reduction must be avoided even though the degree of reduction achieved by the preliminary reduction in the rotary kiln is subject to relatively strong fluctuations. As such, the preliminary reduction is not effected as far as to the FeO state but, for the sake of precaution, only to a much higher degree of oxidation so that a larger reduction work must be performed in the melting process, which is effected in electric furnaces in most cases. As a result, the overall process becomes more expensive. Additionally, control of further reduction during the melting process is difficult because the degree of oxidation and the carbon content of the matter discharged from the rotary kiln often fluctuate, even in the case of small kilns.
Such a process has been described in TMS-AIME Paper Selection, Paper No. 74-40, pages 1-23.
Another case relates to the reduction of ores which contain higher manganese oxides to be reduced to lower manganese oxides.